The present disclosure relates to a master disc used to manufacture optical discs, a recording drive waveform adjusting method to manufacture the master disc, a master disc manufacturing method, and a master disc manufacturing apparatus. Particularly, the present disclosure relates to a technique of obtaining an appropriate recording drive waveform (strategy pattern).
In a process of manufacturing optical discs for reproducing only having a pit array of so-called emboss pits, a master disc having a projection-and-depression pattern corresponding to a pit array is manufactured first. Then, a stamper is formed based on the master disc, and optical discs are mass-manufactured by using the stamper.
For manufacturing the master disc, a master disc manufacturing apparatus, typically called a cutting apparatus or a mastering apparatus, is used.
The master disc manufacturing apparatus allows a master disc substrate on which a resist layer is formed to rotate, and applies laser light collected by an objective lens. At this time, the laser light is focus-controlled so that the laser light follows on the surface of the resist master disc within its focus depth. Also, the laser light is applied onto the resist layer while being controlled to form an exposure pattern corresponding to a pit array according to a recording signal.
After the exposure pattern corresponding to a pit array has been formed in the resist material on the master disc, then development (etching) is performed in a typical case so as to form a pit array of a projection-and-depression pattern by using steps of the resist material.
Japanese Unexamined Patent Application Publication Nos. 2007-48412, 2006-344399, 2005-276437, and 2004-47025, Japanese Patent No. 3881673, Japanese Unexamined Patent Application Publication No. 2004-199848, and International Publication Pamphlet No. 2002/084653 disclose various techniques about correction of write strategy patterns to perform laser drive according to recording data in a recording/reproducing apparatus for a write-once disc or a rewritable disc.
When laser irradiation is performed according to a recording signal in order to manufacture a master disc, an appropriate write strategy is necessary to be set. For example, when a master disc for manufacturing an optical disc having pit lengths of 2T to 9T (T is a channel clock cycle) is manufactured, exposure corresponding to pits of 2T to 9T is performed by laser irradiation. At that time, when a strategy pattern (recording drive waveform applied to a laser light emitting unit) is appropriately adjusted for each pit length of 2T to 9T, the quality of a pit array of optical discs as end products can be maintained.
However, strategy adjustment during manufacturing of a master disc is troublesome and is disadvantageous for increasing efficiency of a process of manufacturing optical discs.
According to related arts, strategy adjustment in a master disc manufacturing apparatus is performed in the following procedure. First, cutting (exposure) of a master disc is performed in a certain strategy pattern setting state, and then development is performed to manufacture a master disc. Furthermore, a stamper is manufactured based on the master disc. Then an optical disc as an end product is manufactured in the same manner as in an ordinary manufacturing process by using the stamper. In this stage, the manufactured optical disc is evaluated. For example, reproducing of the optical disc is performed, jitter and asymmetry of a reproduced signal are measured to evaluate signal quality, and whether a high-quality pit array is formed is checked. Based on the check result, an appropriate condition (correction value) of a strategy pattern in the stage of manufacturing a master disc is calculated and strategy adjustment in the master disc manufacturing apparatus is performed.
In the above-described procedure, the following problems occur.
First, an appropriate strategy condition is calculated after manufacturing an optical disc as an end product, and thus the master disc, stamper, and optical disc that are manufactured in the first stage are used for strategy adjustment. That is, the master disc, stamper, and optical disc are not used as a real product and become unnecessary after strategy adjustment has been done. This causes waste.
Furthermore, even an optical disc is manufactured for strategy adjustment in manufacturing of a master disc, which takes time and is very insufficient. This step of manufacturing an optical disc for strategy adjustment imposes a heavy burden on a process of manufacturing optical discs. Also, it may be impossible to automate the step of manufacturing an optical disc.
Furthermore, if strategy adjustment is not appropriately performed in one step and if the strategy adjustment is repeated several times, the above-described waste (waste of products and process) doubles. In order to avoid such waste, a recording signal as a test pattern that is specialized to easily calculate a strategy condition should be prepared so that appropriate strategy adjustment can be performed in one step. For example, a recording signal including a 2T repetition pattern may be used. For this purpose, operations and an apparatus configuration that are unnecessary in an ordinary master disc manufacturing stage, e.g., setting of a test signal and supply of the test signal for cutting, become necessary.